The demand for increasing areal density in the magnetic storage industry drives the search for new magnetoresistive readers with increased sensitivity. The tunneling magnetoresistive (TMR) read head is one device that has been investigated recently as a highly sensitive magnetoresistive reader. A TMR utilizes a magnetic tunnel junction (MTJ) composed of a tunnel barrier layer made from a non-magnetic insulating material sandwiched between two ferromagnetic layers. The insulating layer is thin enough to permit quantum-mechanical tunneling of charge carriers between the ferromagnetic layers. The tunneling is electron spin-dependent and, therefore, the tunneling current depends on the spin-dependent electronic properties of the ferromagnetic materials and the relative orientations of the magnetization directions of the ferromagnetic layers. For this reason, the two ferromagnetic layers are designed to have different responses to magnetic fields so that the orientation of their magnetic moments may be varied by an external magnetic field. One of the ferromagnetic layers in the MTJ, called the pinned layer, is composed of a material whose magnetic moment does not rotate in response to an applied magnetic field in the device's range of interest. In some MTJs the ferromagnetic layer is pinned by being exchange coupled to an antiferromagnetic layer. The other ferromagnetic layer is a free layer, that is, its magnetic moment is free to respond to an applied magnetic field in the device's range of interest.
Some MTJs include a tunnel barrier layer doped with magnetic particles. This doping provides an increase in magnetoresistance and an improved signal to noise ratio. For example, some MTJs have an aluminum oxide tunnel barrier layer doped with magnetic particles such as cobalt, iron or nickel particles.
The success of the tunneling in an MTJ will depend on the quality of the insulting tunnel barrier layer in the device. A high quality tunnel barrier layer is desirably made from a uniform, stoichiometric oxide composition sandwiched between two completely unoxidized ferromagnetic layers. Unfortunately the production of such high quality tunnel barrier layers has proven to present a significant challenge because it is difficult to completely oxidize the tunnel barrier layer in the MTJ without at least partially oxidizing the neighboring ferromagnetic layers and any magnetic particles used to dope the tunnel barrier layer. In one common fabrication scheme a pure metal is deposited on a ferromagnetic layer and subsequently oxidized in air or in an oxygen gas or plasma. Using this method, the oxidation progress must be precisely monitored in order to avoid over- or under-oxidizing the tunnel barrier layer. In addition, this approach may result in an oxygen concentration gradient across the tunnel barrier layer and the underlying ferromagnetic layer. Such an oxygen concentration gradient may be difficult to eliminate once formed. Another method presently used to produce MTJs is reactive sputtering from a metal target in an oxygen gas atmosphere to deposit a metal oxide onto a ferromagnetic layer. Unfortunately this approach exposes the underlying ferromagnetic layer to an oxygen rich plasma, causing at least a portion of the ferromagnetic material to become oxidized. Sputtering from an oxide target is another technique used to deposit a metal oxide layer on a ferromagnetic layer. However, this approach typically provides low yield and poor reproducibility and is characterized by an inability to consistently form thin, uniform, pinhole-free layers of metal oxide.
In the production of a MTJ it is important to use an insulating tunnel barrier layer that is neither over-nor under-oxidized, as both conditions will affect the performance of the device. The effects of over- or under-oxidation of the tunnel barrier layer may be illustrated using a MTJ with an aluminum oxide tunnel barrier layer as an example. If the pure aluminum (Al) layer is under-oxidized, the unoxidized Al adjacent the neighboring ferromagnetic layer will significantly influence the behavior of tunneling in the device. Specifically, at the ferromagnetic/aluminum interface, the ferromagnetic material may induce spin polarization of the Al layer to an extent that is lower than the polarization of the ferromagnetic material. Thus, the tunneling current coming from the underlying ferromagnetic layer in contact with the Al metal will be only weakly polarized, thereby decreasing the junction magnetoresistance. In addition, the pure Al present in the tunnel barrier layer may react with the underlying ferromagnetic materials (e.g., Co and Fe) to form intermetallic compounds and degrade the performance of the junction. On the other hand, if the tunnel barrier layer is over-oxidized there is a possibility that the oxidation will continue into the underlying ferromagnetic surface. The formation of an oxide of the ferromagnetic material gives rise to spin scattering and an increase in the junction resistance, resulting in a decrease in the junction magnetoresistance.
Thus a need exists for a method of selectively and completely oxidizing the tunnel barrier layer in an MTJ without oxidizing the neighboring ferromagnetic layers to provide a thermodynamically stable MTJ.